| Have you got flying in and out leads or are you racked and patch bayed? It sounds like you may have some intermittant cables or some problem with the LA-3 wiring or internal circuitry. You said you isolated the LA-3 and it was ok. I would try that again with a known good input source and known good monitoring setup. If this is a vintage unit has it ever had its electrolytic caps replaced both in the power supply and signal chain? The frequencies you described sound more like digital noise or machine noise whine rather than ac hum. Faulty caps could be the prob. even if it's intermittant. Your cables should also be tested. Bt the way if your racked the rack itself may be causing intermittant ground loops between your units that are in that rack.
gb
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